1. Field of the Invention
The present invention relates to a structure of a seat back with lumbar support for a vehicle.
2. Description of the Related Art
In a vehicle, and in particular an automobile, a headrest is provided in the upper part of a seat back as a countermeasure against neck injury (whiplash) in a vehicle occupant in the event of a rear collision. However, a headrest alone is not sufficient as a countermeasure against neck injuries.
Therefore, it has been proposed to provide a coupling member which extends rearwards only when a load equal to or exceeding a prescribed value is applied, in the coupling portion of a seat back frame with an S spring (a cushion spring which is bent in a continuous plurality of S shapes) (see, for example, Japanese Patent Application Laid-open No. H7-291005).
When the weight of a vehicle occupant acts on an S spring as a reaction to a rear impact, the coupling member extends rearwards, the occupant's back sinks into the seat back, and hence the occupant's head is received and held by the headrest.
On the other hand, there have also been proposed to provide a lumbar support plate for supporting an occupant's lumbar region, inside a seat back (see, for example, Japanese Utility Model Application Laid-open No. H4-107651).
More specifically, as shown in FIG. 7 and FIG. 8, a lumbar support plate 1 which is supported movably in the front/rear direction is provided inside a back frame of a seat back (see reference numeral 20 in FIG. 1).
A clutch mechanism 4 which is capable of rotating a pinion 3, which forms an output member, by means of a rotating operation of a handle shaft 2, is provided in one side frame of the back frame (see reference numeral 20b in FIG. 1). Furthermore, a torsion bar 5 is provided as a biasing member which biases the lumbar support plate 1 forwards, one end portion 5a of the torsion bar being supported by the other side frame of the back frame (see reference numeral 20c in FIG. 1) and the other end portion 5b of the torsion bar being fixed to a fan-shaped gear (turning member) 7 which is turned by the pinion 3 of the clutch mechanism 4.
As a member for adjusting the front/rear position of the lumbar support plate 1, the front/rear position of the other end portion 5b of the torsion bar 5 is adjusted by turning the fan-shaped gear 7 in relation with the rotation of the pinion 3 of the clutch mechanism 4.
As shown in FIG. 9A, the clutch mechanism 4 (also called a brake mechanism) comprises a brake drum 10, a brake spring 11 which makes pressure contact with the inner wall of the brake drum 10, a core 12 which is formed integrally with the handle shaft 2, and a hook section 3a which is formed integrally with the pinion 3 (see, for example, Japanese Utility Model Application Laid-open No. H7-19562).
As shown in FIG. 9B, the side end portion of the hook section 3a presses a hook section 11a (or 11b) of a brake spring 11 so as to increase the outer diameter of the brake spring 11, against the rotational force from the pinion 3 (the lumbar support plate 1). Consequently, the pressure contact force between the brake spring 11 and the brake drum 10 becomes stronger and rotation of the pinion 3 is impeded (braked state).
Furthermore, in response to a rotational force from the handle shaft 2 side, the hook section 11a (or 11b) of the brake spring 11 receives force in a direction which compresses the outer diameter from the side end portion 12a of the core 12. Therefore, the pressure contact force becomes less, the handle shaft 2 becomes rotatable (brake released state), and the pinion 3 can be rotated via the brake spring 11 and the hook section 3a. 
As shown in FIG. 8, in the clutch mechanism 4, the brake spring 11 and the core 12 are incorporated inside a brake drum 10, and the brake drum 10 is fixed by screws 15 to a bracket 14. Furthermore, the handle shaft 2 is coupled to a core 12, and the hook section 3a of the pinion 3 is fitted inside a slot 12a in the core 12 from the outer side of the bracket 14.
The upper portion of the fan-shaped gear 7 which meshes with the pinion 3 is fixed turnably by a pin member 16 via a plate spring 25 to the upper part of the outer surface of the bracket 14, and the other end portion 5b of the torsion bar 5 is fixed by screws 18 to the fan-shaped gear 7.
However, in Japanese Patent Application Laid-open No. H7-291005, an S spring is simply provided in the seat back frame, and this S spring has absolutely no lumbar supporting function. Furthermore, in Japanese Utility Model Application Laid-open No. H4-107651, there is a lumbar supporting function, but no countermeasure against neck injury is provided.
Therefore, in order to provide both a lumbar supporting function and a countermeasure against neck injury, it is necessary to provide both a composition having a lumbar supporting function and a composition having a countermeasure against neck injury, and therefore the number of components increases, costs also rise, and the weight also increases.